Call Lightning Spell: Does it provoke additional attacks of opportunity every use?

[Lord Tsarkon]

Call Lightning

Immediately after casting this spell you can take a standard action(concentrating on a spell) to call a bolt..(1 bolt/ level usually)

I understand the first round is like casting a spell and so provokes, but do the additional bolts in subsequent rounds also provoke attacks of opportunity?

Can you cast Defensively the additional bolts to not provoke(if it does provoke at all)? What happens if you fail the check, do you lose the spell as it is written?

Casting on the Defensive: Casting a spell while on the defensive does not provoke an attack of opportunity. It does, however, require a concentration check (DC 15 + double the spell's level) to successfully cast the spell. Failure means that you lose the spell.

[Dennis Baker]

The first bolt is a free action.

Then: "Each round after the first you may use a standard action (concentrating on the spell) to call a bolt. "

You are not casting each round, merely concentrating on the spell which does not provoke.